Archive for March, 2011
Here are some tips about Metric Speak that all PCB designers need to know. “Metric” is not a unit of measure. Metric is a term that describes a measurement system. You use either millimeters or microns for your PCB design units. The proper terminology to describe your working units when using the metric measurement system is millimeters or microns, not metric. Example: When doing PCB layout in Inches or Mils you never refer to working in “Imperial Units”.
Millimeters allow finer (and greater) granularity in the PCB design grid system to optimize board real-estate, part placement, via fanout and routing trace/space features and snap grids. This will be very important in the future of PCB RF Micro-technology. PCB impedance measurements are more accurate in Micron units than “Ounces of Copper” and Mil core/Prepreg dielectric. Use Micron Units to achieve the highest level of accuracy for impedance calculations.
Unfortunately, PCB manufacturers are directly responsible for holding back the progress of the transition to metrication of our industry. When the PCB fabrication companies transitions to the metric system, the entire electronics industry will achieve the peak of “electronic product development automation”. Until then, we’ll plod along using dual units in the land of chaos.
Here is an example of the chaos in the Chip Component family. All Chip names refer to their body length and width. When EIAJ introduced the standard Chip and Molded body component dimensions, only millimeter units were used. A 3216 was 3.2 mm long and 1.6 mm wide. It was very simple. When the data was passed on to EIA in America, they changed all the chip names from millimeters to Inches and a 3216 was renamed 1206 or 0.125” length and 0.062” width (just drop the 3rd place number). Today most component manufacturers dimension all there component packages in millimeters see Table 1 that illustrates Metric vs. Imperial names. You can easily see the confusion in the dual measurement system.
Let’s start the transition process. 99% of all PCB layouts use vias. See Table 2 for an Inch to Millimeter chart for common via sizes starting with a 0.15 mm hole and growing in 0.05 mm increments. I’ll provide the entire padstack conversion. I intentionally did not add thermal relief data because vias should have a direct plane connection (no thermal relief is necessary). When transitioning from Imperial units to Metric units, always round-off the millimeter values in 0.05mm increments for normal resolution. If you’re working on extremely dense hand held device technology, round-off to the nearest 0.01 mm. For PCB design, there is no reason to go more than 2 places to the right of the decimal point for the present. 0.01 mm = 0.0003937”
Table 3 illustrates 4 common inch based part placement grids and their millimeter equivalent. The common rule in placing parts in millimeters is to always stay one place to the right of the decimal or 0.1 mm increments.
Table 4 provides all the common trace/space technology and routing snap grids. The common rule when working in millimeters is to always use a 0.05 mm routing grid. Most component lead pin pitches are 0.05 mm increments and IPC-7351B land (pad) sizes and snap grids are in 0.05 mm increments. This totally optimizes trace routing and eliminates wasted PCB real-estate. Everything fits together tighter than Lego building blocks. Notice that in the inch units, a gridless shape-based option is used, but in millimeters all objects can easily snap to a grid and still achieve maximum density solutions. I provide 3 various route snap grid solutions for the various trace/space rules.
Note: Inch based routing grids are evenly divisible into 0.100” while millimeter based routing grids are evenly divisible into 1 mm.
Table 5 provides the PCB material equivalents. Note that the various columns are not related to each other. Each column describes a specific PCB feature. In the first column “Board Thickness” is common PCB finished material thicknesses and the metric equivalent rounded off to the nearest 0.1 mm. The second column is copper weight in ounces and their micron equivalent. Using microns to describe copper thickness is better than using weight. The third and forth columns go together. Column 3 defines the type of hole and column 4 provides the PCB fabrication tolerance for each different hole type in the chart.
Table 6 is common plated through-hole padstacks for component leads and their inch to millimeter conversion. All hole, pad and plane clearance values are in 0.05mm increments. The Solder Mask is the same value as the outer layer pads. This padstack information was taken from the proportional padstack table and you can download it here under “Appnote 10836: Proportional Through-hole Padstacks” – http://www.mentor.com/products/pcb-system-design/library-tools/lp-wizard/import-docs
Note: this downloadable chart only contains millimeter values and not the inch equivalents in Table 6.
Table 7 is common non-plated through-hole padstacks and their inch to millimeter conversion. All hole, pad and plane clearance values are in 0.05mm increments. The Solder Mask is the same value as the hole size to allow the PCB manufacturer to oversize it per their specific fabrication tolerances. Notice that the pad size for every padstack is 1.00 mm. Because the holes are not plated, the hole size is typically larger than the hole size. Also, there is no reason to have multiple pad sizes when the pad is eventually drilled away. The only reason for having a pad in a non-plated padstack is display a marker as a guide for the hole location. The PCB manufacturer does not need the pad in the padstack, but sometimes when there is no pad (but there is a drill hole) the manufacturer might question if the hole is valid. Of course there is no thermal relief required in non-plated hole padstacks.
I want to note that the LP Calculator automatically performs all of these through-hole padstack calculations for you and provides 5 different options –
IPC-7251 Most Environment
IPC-7251 Nominal Environment
IPC-7251 Least Environment
User Defined Environment Rules
You can get a free LP Calculator by signing up for a 10-day evaluation of LP Wizard here – http://www.mentor.com/products/pcb-system-design/library-tools/lp-wizard/lp-wizard-eval
After the LP Wizard 10-day evaluation is over, the LP Wizard program will run in “Demo Mode” as LP Calculator.
If the French would have won the French & Indian War against the British (the 7 Year War from 1754 to 1763) Imperial Units or the English measurement system would not exist in society today. Even the British transitioned to the metric measurement system 46 years ago. America is the last stronghold for the Imperial measurement system and how much longer will it take for the world to become united under a single measurement system. This is what world standards and space age technology acceleration will require to fully automate all PCB processes.
One of the greatest secrets to PCB design perfection today in 2011 is the use of the metric unit system. From 1974 – 1991 we used Inch units for PCB layout. From 1991 – 2001 we used Mil units. From 2001 – 2011 we used millimeter units. I have to say that when we made the transition from Mils to millimeters our productivity levels slipped a bit during the learning curve. But after 5 or 6 PCB layouts our productivity was back to normal. After about 15 PCB layouts our productivity levels surpassed all previous results. If I was forced to go back to the Mil measurement system, my productivity levels would reverse backwards. There is no way in the world that anyone in 2011 using Mil units can outperform the same talent using Millimeter units because most component pin pitches are on a millimeter grid system (like the 1 mm pitch BGA) and metric units are vastly superior to work within the PCB design space because all the numbers are evenly divisible by 10 and there is no use for calculators for mathematical calculations. There is no one that I know of that has successfully transitioned to the metric unit system for PCB layout that wants to go back to the Imperial unit system. That statement alone tells it all.
As a matter of fact, there would not be Imperial units in the world today if the United States government (congress) fulfilled the commitment that they signed at the Treaty of the Meter back in 1875. I hear it all the time from corporations who will not convert – “We’re American and we have our own measurement system. We are not part of the European Union or Russia or Japan. We’re proud to be Americans and we believe in our way of life and the system and values that we use”. Well, let me shine a little light on all those proud Americans who obviously do not know the historical facts. So before I go into PCB design details of why metric units are superior, I need to explain the historical background to set the stage.
Most Americans think that our involvement with metric measurement is relatively new. In fact, the United States has been increasing its use of metric units for many years, and the pace has accelerated in the past four decades. In the early 1800′s, under the presidency of Thomas Jefferson, the U.S. Coast and Geodetic Survey (the government’s surveying and map-making agency) used meter and kilogram standards brought from France. Abraham Lincoln was a strong proponent of the metric unit system and in 1866 (just 1 year after his assassination), Congress authorized the use of the metric system in America and supplied every state with a set of standard metric weights and measures.
In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Treaty of the Meter. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Treaty of the Meter, also known as the “Metric Convention” established the International Bureau of Weights and Measures (BIPM) in Sèvres, France, to provide standards of measurement for worldwide use.
In 1893, metric standards, developed through international cooperation under the auspices of BIPM, were adopted as the fundamental standards for length and mass in the United States. Our customary measurements — the foot, pound, quart, etc. — have been defined in relation to the meter and the kilogram ever since. The General Conference of Weights and Measures, the governing body that has overall responsibility for the metric system, and which is made up of the signatory nations to the Treaty of the Meter, approved an updated version of the metric system in 1960. This modern system is called Le Système International d’Unités or the International System of Units, abbreviated SI.
The United Kingdom began a transition to the metric system in 1965 to more fully mesh its business and trade practices with those of the European Common Market. The conversion of the United Kingdom and the Commonwealth nations to SI created a new sense of urgency regarding the use of metric units in the United States.
In 1968, Congress authorized a three-year study of systems of measurement in the U.S., with particular emphasis on the feasibility of adopting SI. The detailed U.S. Metric Study was conducted by the Department of Commerce. A 45-member advisory panel consulted with and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials.
The final report of the study, “A Metric America: A Decision Whose Time Has Come” concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement. The study found that measurement in the United States was already based on metric units in many areas and that it was becoming more so every day. The majority of study participants believed that conversion to the metric system was in the best interests of the Nation, particularly in view of the importance of foreign trade and the increasing influence of technology in American life.
The study recommended that the United States implement a carefully planned transition to predominant use of the metric system over a ten-year period. Note: In 1975, the Australian continent also implemented its metric conversion act and successfully transitioned. The United States Congress passed the Metric Conversion Act of 1975 “to coordinate and plan the increasing use of the metric system in the United States.” The Act, however, did not require a ten-year conversion period. A process of voluntary conversion was initiated, and the U.S. Metric Board was established. The Board was charged with “devising and carrying out a broad program of planning, coordination, and public education, consistent with other national policy and interests, with the aim of implementing the policy set forth in this Act.” The efforts of the Metric Board were largely ignored by the American public, and, in 1981, the Board reported to Congress that it lacked the clear Congressional mandate necessary to bring about national conversion. Due to this apparent ineffectiveness, and in an effort to reduce Federal spending, the Metric Board was disestablished in the fall of 1982.
The Board’s demise increased doubts about the United States’ commitment to metrication. Public and private sector metric transition slowed at the same time that the very reasons for the United States to adopt the metric system — the increasing competitiveness of other nations and the demands of global marketplaces — made completing the conversion even more important.
Congress, recognizing the necessity of the United States’ conformance with international standards for trade, included new encouragement for U.S. industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designates the metric system as the preferred system of weights and measures for United States trade and commerce.” The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement.
Federal agencies were required by this legislation, with certain exceptions, to use the metric system in their procurement, grants and other business-related activities by the end of 1992. While not mandating metric use in the private sector, the Federal Government has sought to serve as a catalyst in the metric conversion of the country’s trade, industry, and commerce.
The current effort toward national metrication is based on the conclusion that industrial and commercial productivity, mathematics and science education, and the competitiveness of American products and services in world markets, will be enhanced by completing the change to the metric system of units. Failure to complete the change will increasingly handicap the Nation’s industry and economy.
There is one thing that I would like to clarify to the reader that I’m not proposing that the American “way of life” change in our sports (football, baseball, golf, etc.) or cooking units in our kitchens, but rather our “industry” must change to increase our competitiveness with the rest of the world. However, America has an impact on other counties weights and measurement systems. The EU Metric Directive (80/181/EEC), that was scheduled to go into effect on January 1, 2010, has been modified to allow the continuation of both supplemental (U.S. customary, inch-pound) and metric units for consumer goods sold in the EU. The rule was published on May 7, 2009 in the Official Journal of the European Union.
The modified Directive instructs the European Commission to produce a report to the Parliament and Council regarding the smooth functioning of the internal market and international acceptance of SI units by December 31, 2019, including proposals where appropriate. Demonstrated progress will be important to achieve long-term acceptance of supplemental units in the EU. Modifying the U.S. Fair Package and Labeling Act (FPLA) to permit metric labeling is an example where greater international marketplace acceptance of SI units can be achieved.
Next week I will present Imperial to Metric conversion charts as they apply to the PCB design industry. I will also post a short message on the proper terminology that I refer to as “Metric Etiquette”.
Mounting hardware normally consists of these 4 items (See Figure 1) –
- Phillips Head Screw
- Hex Nut
- Flat Washer
- Lock Washer
There are 4 types of mounting holes –
Supported – Plated through with annular ring
Supported – Plated through with annular ring with vias
- Unsupported – Non-plated and with copper pads
Unsupported – Non-plated and with no copper pads
The supported mounting hole usually gets tied to the GND plane without a Thermal Relief (a direct connection is best) and the supported hole w/vias gets both the main hole and the vias tied to the GND plane. Due to the fact that mounting hardware never gets soldered to the PCB, there is no reason for a Thermal Relief pattern and you connect all holes (including vias) directly to the plane. The unsupported (non-plated) hole has no connection to a GND plane layer and they require an outer layer keep-out defined that compensates for the hardware tolerances. See figure 2 for an illustration of the slop tolerance of a flat washer and the necessary copper keep-out sizing.
There are two primary reasons for adding vias to the supported mounting hole. The first was to insure that if the screw threads stripped the copper plating from the main hole that the vias would still provide adequate ground connections. The second reason was for additional support to prevent the PCB from crushing when too much torque was used to tighten the nut. The average via hole size for mounting holes is 0.5 mm. See Figure 3 for a supported mounting hole with vias.
See Table 1 for the most popular PCB hardware sizes for metric unit technology.
In Tables 2 and 4 there are 3 different padstack configurations for each metric screw size for land (pad) size calculations.
- No Washer – Pan Head Clearance
- Flat Washer
The land (pad) diameter is equal to the hardware diameter and a placement courtyard is added to compensate for the slop tolerance indicated in Figure 2.
Note: These Land (pad) and Placement Courtyard padstack values are in the “Least” material values. You can add 0.25 mm for “Nominal” or 0.5 mm for “Most” Land (pad) and Placement Courtyard environments. The hole sizes are for a loose fit.
See Table 3 for the most popular PCB hardware sizes for ANSI standards.
The “Loose Fit” mounting holes are normally used on large boards greater than 100 mm (4”) and the “Tight Fit” mounting holes are commonly used for smaller board sizes.
There are some differences in hardware manufacturer’s feature sizes, so make sure that the hardware you use is adequately covered with the correct pad size and/or keep-out.
There are 3-Tiers for the Mounting Hole family, but the only difference is the “Placement Courtyard Excess”:
Least – 0.1 mm annular
Nominal – 0.25 mm annular
Most – 0.5 mm annular
Note: All numeric values in the Tables are in millimeters
About Tom Hausherr's Blog
New component package technology and CAD library standards.
- PCB Design Perfection Starts in the CAD Library – Part 19
- PCB Design Perfection Starts in the CAD Library – Part 18
- PCB Design Perfection Starts in the CAD Library – Part 17
- PCB Design Perfection Starts in the CAD Library – Part 16
- PCB Design Perfection Starts in the CAD Library – Part 15 QFN
- Inch to Metric Conversion Tables for PCB design
- June 2011 (3)
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